The present invention relates to a data transmission system for effecting data transmission in a contactless manner between units which are arranged at separate positions by using a spread spectrum communication technique and, more particularly, to a data transmission system for effecting data transmission between units by using two kinds of M-series signals.
In recent years, in association with the practical use of factory automation systems, there has been considered a system in which working programs, working data, and the like which are used in a controller of a machining center or the like are stored in a memory module provided in a pallet to which a workpiece is attached, and when the pallet is conveyed to the machining center, the content of the memory module is automatically read out and loaded into the controller.
It is preferable that the above noted data transmission system for use in a factory automation system is constructed as a contactless transmission system. For this purpose, three kinds of contactless transmission systems such as radio wave systems, photo coupling systems, and electromagnetic induction coupling systems have been proposed.
However, in the radio wave system, since microwaves are used, the installation conditions of peripheral apparatuses are limited due to a problem of the reflection or the like. In the photo coupling system, there is a problem of fouling by an oil or a dust. Thus, in recent years, attention has been paid to the electromagnetic induction coupling system which can be stably used at a location having severe environmental conditions.
However, in the electromagnetic induction coupling system in which the data transmission is effected between two units by disposing the induction coils provided in the two units near each other, since the system is what is called a transformer coupling type system, the magnetic force deteriorates in inverse proportion to the cube of the distance. Thus, if the interval between the coils is not reduces to a few millimeters of less, stable data communication in a factory where a large amount of external noise exist cannot be guaranteed. With respect to a point that the two units must be disposed near each other until the distance there-between is about a few millimeters, such a drawback is significant in the electromagnetic induction coupling system as compared with the radio wave system and the photo coupling system in which the transmission distances can be set to be relatively large and become a cause of the delay of the realization of the practical use.
Therefore, the inventors of the present invention have proposed a system for significantly increasing the transmission gap interval by applying a spread spectrum communication technique to the electromagnetic induction coupling system in U.S. patent application Ser. No. 07,387,966 (1989).
For instance, two kinds of M-series generators are prepared on the transmission side and different M-series signals are transmitted in accordance with the data bits 0 and 1. On the reception side, the two M-series signals on the transmission side are stored as reference values in a memory and after the reception signals were sampled at a predetermined period, the correlation calculations are sequentially executed in parallel between the sampled reception signals and each of the two M-series reference values. Then, the two calculated correlation values are compared. Since the correlation value in which the reception signal and the series of the reference value coincide is larger than the correlation value in which the reception signal and the reference value series differ, the data bit 0 or 1 corresponding to the reference value used in the calculation of the larger correlation value is output.
In a data transmission system in which the presence or absence of the auto-correlation between the reception signal and the reference values is calculated by using the two kinds of M-series signals as mentioned above, the correlation value in the case where any one of the signal arrangements has deviated is much smaller than the correlation value in the case where the arrangements coincide between the same two M-series signals. The S/N ratio of the reception signal when they coincide with the reception signal when they differ is fairly high. That is, in the M series of a word length of 2.sup.N -1, when the series has deviated by one stage, the correlation value is reduced to -1/(2.sup.N -1).
However, since the mutual correlation of the two kinds of M-series signals is calculated with respect to the reception signal of the M-series which is different from the reference value, the derived correlation value is dependent upon the series position. A satisfactory S/N ratio in the auto-correlation is not guaranteed.
On the other hand, since two kinds of M-series signals are generated in correspondence with the data bits 1 and 0, there is a problem in that two M series generating circuits are necessary and the circuit construction also becomes complicated.